By an electrophotographic system, a visible image is formed usually in such a manner that firstly an electrostatic latent image is formed on a photoconductor drum, and then, it is developed with a toner, then transferred to e.g. transfer paper and fixed by e.g. heat. As such a toner for developing an electrostatic charge image, it is common to use one having an auxiliary agent deposited for the propose of imparting various properties such as flowability, on the surface of toner host particles obtained by a so-called melt-kneading pulverization method i.e. a method wherein a binder resin and a colorant are dry-blended with an antistatic agent, a release agent, a magnetic material, etc. as the case requires, and the mixture is melt-kneaded by e.g. an extruder, followed by pulverization and classification.
In recent years, a high image quality is regarded as a performance desired for printers and copying machines, and in order to accomplish such a high image quality, the above-mentioned toner host particles are required to have a volume average particle diameter at a level of from 4 to 8 μm and a narrow particle diameter distribution. However, in the above-mentioned melt-kneading pulverization, it is difficult to control the particle diameters of the toner host particles, and if it is attempted to obtain toner host particles having a volume average particle diameter at a level of from 4 to 8 μm, a large amount of fine powder having a particle diameter below the desired level is likely to be naturally formed, and there has been a problem that it is difficult to separate such a fine powder in the classification step.
As a method for overcoming the above problem in the melt-kneading pulverization method, (a) a suspension polymerization method wherein a polymerizable monomer, a polymerization initiator, a colorant, etc. are suspended and dispersed in an aqueous medium, followed by polymerization to prepare toner host particles, (b) an emulsion polymerization flocculation method wherein a polymerizable monomer is emulsified in an aqueous medium containing an polymerization initiator, an emulsifier, etc., and the polymerizable monomer is polymerized with stirring to obtain a dispersion of polymer primary particles, then a colorant, etc. are added to the dispersion, followed by flocculation and aging of the polymer primary particles to prepare toner host particles, or (c) a dissolution suspension method wherein a dissolution dispersion (dissolution dispersion of toner composition) having a polymer, a colorant, etc. preliminarily dissolved or dispersed in a solvent, is dispersed in an aqueous medium, and the solvent is removed by e.g. reducing the pressure for heating the dispersion to prepare toner host particles.
By such wet methods, it is possible to obtain toner host particles having small particle diameters and relatively narrow particle diameter distribution. However, simply by adopting such methods, no adequate results have been obtained for controlling the shape of the toner host particles or for making the particle diameters to be uniform.
Further, for copying machines or printers of recent years, still higher levels of image characteristics and image quality are required, and a toner is required which presents no adverse effects with respect to scratching, abrasion, etc. also against components constituting an image-forming device. For such a purpose, a higher level of control of the particle diameter of the toner is required, and it is particularly important that a coarse (large particle diameter) toner is less.
If toner host particles are prepared by such wet methods, it is certainly possible to obtain ones having a low content of coarse particles as compared with toner host particles prepared by the melt-kneading pulverization method, but even by such wet methods, it was not possible to sufficiently reduce coarse particles. Various reasons may be considered, but in the case of toner host particles by wet methods, agglomeration of toner host particles one another in a state dispersed in an aqueous medium or deposition in the container during polymerization or dissolution is, for example, considered to be a cause.
Therefore, heretofore, it has, for example, been common that as shown in Patent Document 1, toner host particles are dried, and an auxiliary agent is deposited on the surface of the toner host particles, and then coarse particles in the toner host particles are removed by screening. However, merely by such a dry screening step after addition of an auxiliary agent, clogging of the screen is likely to take place, whereby the yield is poor, and the process is not efficient, and further, by the presence of coarse particles at the time before the addition of the auxiliary agent to the toner host particles, there has been a problem that the agglomeration is accelerated at the time of addition of the auxiliary agent.
On the other hand, in the production of toner host particles by wet methods, the process may have a filtration step, and such a filtration step is intended to remove a suspension stabilizer or emulsifier, impurities or by-products, or fine particles with small particle diameters, and it has been common not to include a step intended to remove coarse particles larger than the toner host particles.
Under the circumstances, Patent Documents 2 and 3 disclose a process for removing coarse particles in such toner host particles. However, the techniques disclosed in such Patent Documents have had a problem that when a toner dispersion is supplied continuously, the screen mesh tends to be clogged with coarse particles, and in the step of removing coarse particles, the toner dispersion is likely to overflow from the screen.
Further, Patent Documents 4 to 6 disclose techniques to solve such clogging of coarse particles. However, even by such techniques, although very large particles as compared with the screen mesh may be continuously removed, particles having particle diameters close to the mesh size or particles corresponding to the mesh size still tend to clog the mesh, whereby it has been difficult to carry out screening continuously.
Therefore, as disclosed in Patent Document 7, a screening method has been developed to avoid clogging by supplying the dispersion of toner host particles inversely, but this method tends to lead retention of toner host particles and is not a productive method.
Thus, heretofore, it has not been clearly understood how to remove coarse particles efficiently from the toner by wet methods and to obtain a toner for developing an electrostatic charge image excellent in image characteristics, image quality, scratch-resistance of a developing device, etc., and no screen device has been available to accomplish it.
Patent Document 1: JP-A-2001-249491
Patent Document 2: JP-A-2000-172007
Patent Document 3: JP-A-2002-196534
Patent Document 4: JP-A-2004-198793
Patent Document 5: JP-A-2004-245990
Patent Document 6: JP-A-2005-193155
Patent Document 7: JP-A-2004-337784